1. Field of the Invention
The present invention relates in general to an ocular lens such as a contact lens placed on an eyeball or an intraocular lens inserted within the eye. More particularly, the invention is concerned with a multifocal ocular lens having a plurality of vision correction regions with different optical or vision correction powers.
2. Discussion of the Related Art
As an ocular lens used for vision correction of an eye having weakened accommodation faculty such as presbyopia, there has been proposed a bifocal or multifocal ocular lens having a plurality of vision correction regions which provide respective different optical powers. JP-A-63-95415 and JP-A-1-319729 disclose an alternating or translating vision type contact lens as one example of the multifocal ocular lens, wherein the vision correction regions with different optical powers are selectively and alternatively used as needed with a shift of the visual axis of the lens wearer. U.S. Pat. No. 4,580,882 and JP-A-2-217818 disclose a simultaneous vision type contact lens as another example of the bifocal or multifocal ocular lens, wherein the vision correction regions with different optical powers are simultaneously used, and a desired image observed through one of the vision correction regions is selected by an action of the wearer's brain.
If either of the alternating and simultaneous vision types described above, the vision correction regions of the bifocal lens consist of a near vision correction region through which near objects are observed, and a distant vision correction region through which distant objects are observed. On the other hand, the vision correction regions of the multifocal lens consist of such near and distant vision correction regions, and at least one intermediate region which is interposed between those near and distant vision correction regions and which provides an optical power different from the optical powers of the near and distant vision correction regions.
The bifocal ocular lens having the two vision correction regions, i.e., the near and distant vision correction regions, tends to suffer from a jump or overlapping of the image of an object located intermediate between near and far objects. This is because the bifocal ocular lens has only two focal points which respectively correspond to the near and distant vision correction regions. In this case, the obtained image is undesirably blurred. Further, the bifocal lens has surface discontinuity at a junction of the near and distant vision correction regions, in other words, the two vision correction regions are not smoothly connected to each other at the junction therebetween, deteriorating a wearing comfort as felt by the user.
As compared with the bifocal lens described above, the multifocal ocular lens having the intermediate region(s) between the near and distant vision correction regions provides a clearer image for the object located intermediate between the near and far objects. In the multifocal ocular lens, however, each of the plurality of vision correction regions inevitably has a small radial dimension, so that the optical power of the lens changes in steps in its radial direction, corresponding to the plurality of vision correction regions. In this case, the near and distant vision correction regions do not have sufficiently large surface areas, whereby the images to be obtained through these near and distant vision correction regions undesirably tend to be blurred. Moreover, the optical power in the multifocal lens changes in steps abruptly at each boundary between the adjacent two vision correction regions, in other words, the surfaces of the plurality of vision correction regions are not smoothly connected to one another with a continuous change of the optical power. Therefore, the multifocal lens also suffers from deteriorated wearing comfort. Further, the multifocal ocular lens tends to suffer from problems of so-called "ghosting" (ghost images) or double imaging, and mutual interference of the images obtained through the near and distant vision correction regions.
The assignee of the present invention proposed in JP-A-5-181096 a multifocal ocular lens having a near vision correction region, an intermediate region, and a distant vision correction region, which are located concentrically or coaxially with one another. In the proposed lens, the optical power continuously changes along a suitable curve from the value of one of the near and distant vision correction regions to the value of the other region. According to this arrangement, the optical power changes smoothly even at the junction of the near vision correction region and the intermediate region, and at the junction of the intermediate region and the distant vision correction region, and the lens has a smooth surface configuration which assures the user of a comfortable wearing, without having any surface discontinuity at the junctions. Further, the proposed multifocal ocular lens is free from the problem of ghosting experienced in the conventional multifocal lens wherein the vision correction regions are not smoothly connected to one another.
As a result of an extensive study by the inventors of the present invention, it was found that even the proposed multifocal ocular lens does not sufficiently meet the users' requirements. Described in detail, the proposed lens is still unsatisfactory in providing sufficiently clear viewing of both of the near and distant objects when the lens is used in specific conditions or environments by a painter and a surveying engineer, for instance.